Shorebird Migration at Norman, Oklahoma: 1961-63
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SHOREBIRD MIGRATION AT NORMAN, OKLAHOMA: 1961-63 LEWIS W. ORING AND W. MARVIN DAVIS EPEATED censusing of a limited area provides a valuable source of quanti- R tative migration data despite the many variables involved. Though several such studies of shorebirds have been conducted on or near the Atlantic and Pacific coasts (Urner, 1929; Urner and Storer, 1949; Storer, 1951; Dyke, 1955) no comparable data are available from the central flyway region. This report is a result of observations made from 1 March 1961 to 15 No- vember 1963 at a small impoundment in Norman, Oklahoma. DESCRIPTION OF AREA Norman, Oklahoma is adjacent to the floodplain of the Canadian River. The river flows generally from west to east across the state, but follows a northwest to southeast direction for a considerable distance on both sides of Norman. Native woody vegetation near the city is limited to areas along the river and other streams. Max Westheimer Airfield was developed as a military base in 1942, and is presently operated by the University of Oklahoma. More than 200 acres of asphalt remain, considerable portions of which are not used as runways or taxiways. One such unused area has been the site of a pond since 1949 or 1950. At that time a drainage ditch was dammed in the pasture adjacent to the west edge of the airfield. Except for periods of extreme drought, the pond extends from the pasture onto the airfield as a shallow pool having a maximum depth of about two feet, a length of about 300 yards, a width of about 70 yards, and an area of about three acres. Maximum size is not regu- larly maintained, however, as an extremely high evaporation rate prevails in Oklahoma from April to October. During these months of 1961-63 the average dimensions of the pond were 150 x 40 yards and the average surface was 1.4 acres. The broad, shallow portion of the impoundment, in contrast to the deeper and narrower section in the pasture, has proved quite attractive to migrant shorebirds despite its small size and lack of sandy or muddy shores. When first laid, the asphalt that underlies and nearly surrounds the pond constituted a primary bare area. This area has been invaded for some years by a vigorous plant growth distributed linearly along the seams and cracks of unused areas. Such growth invades the pond at a few points. For about 100 yards along the southwest shore of the pond the asphalt was broken up and hauled away several years prior to this study. This section constitutes the 166 Oring and SHOREBIRD MIGRATION Davis 167 major site of both submerged and emergent vegetation and is the only area of the pond in which an automobile cannot be driven. COVERAGE Censuses were usually taken at least once daily from 1 March to 31 October of all three years. At times, however, a day was missed and on three occasions (1-8 April 1961; 25-30 August 1962; and 26 August to 3 September 1963) both observers were absent from the study area for about a week. From 1 March 1961 to 31 May 1961 daily visits were made at 0600 and lSO0 CST. Throughout the remainder of the study the time of day at which observations were made was variable. Observation periods averaged about an hour, but some lasted from three to six hours in May and September. INFLUENCE OF WEATHER During the frequent spring periods of clear skies and strong southerly winds we experienced a rather uniform pattern of migration and found few rare species, whereas at times of heavy cloud cover, fog, or northerly winds, migrants were grounded and we regularly found uncommon species and larger numbers of individuals. It seems likely that a relatively constant flow of shorebirds passes through central Oklahoma during the height of the north- bound migration but that many of these birds are seen only when forced to the ground by inclement weather. Occasionally an extended wet spell occurred in spring. Concurrently there was a reduction in the shorebird population due apparently to two factors: (1) an increase in feeding sites caused by accumulation of water in fields and on the runway, and (2) the reduction of available food at the pond due to the elevated water level. During very heavy rain or hail only the largest shorebirds remained at the pond-others went to less exposed areas away from the water. Shelter was also sought when the wind velocity reached about 30 mph. At these times we found large flocks of many species running about far from the water feeding voraciously on grounded insects and seeds. Summer in Oklahoma is characterized by an almost continuous span of variably intense southerly winds and clear skies. During such periods the number of southbound migrants changed little from day to day. But when the first cold snap arrived with stron g northerly winds, usually sometime in September, a great variety of shorebirds suddenly appeared; and this phe- nomenon was repeated to a lesser degree with each ensuing north wind until the supply of migrants was exhausted sometime in November. Our observa- tions of fall migrants are thus consistent with the findings of Brooks (1965) THE WILSON BULLETIN June 1966 168 Vol. 78, No. 2 for shorebirds and Hassler et al. (1963) f or p asserines in that they considered the shift of wind to the north to be the primary weather stimulus for south- bound migration. The paucity of migrants during the days just after an extended stormy period was also notable. In sprin g this may have been due to the reluctance of birds to leave the Gulf Coast or other areas of concentration during times of weather disturbance. In summer and fall it may have been due to the small number of birds physiologically ready to migrate that still remained at northern “staging points” following a storm front. RESULTS Thirty of the 39 species of shorebirds recorded from Oklahoma were seen in the study area. Of the nine unrecorded species (Piping Plover, Charudrius melodus; Mountain Plover, Eupoda montana; Ruddy Turnstone, Arenaria interpres; Woodcock, Philohela minor; Eskimo Curlew, Numenius borealis; Knot, Calidris canzztus; Short-billed Dowitcher, Limnodromus griseus; Black- necked Stilt, Himantopus mexicanus ; and Red Phalarope, Phalaropus fuli- carius), the Knot and Red Phalarope have never been collected in Oklahoma and only the turnstone is of more than accidental occurrence in the central part of the state. The Whimbrel (Numenius phaeopus) , White-rumped Sandpiper (Erolia fuscicollis) , Marbled Godwit (Limosa fedoa) , Hudsonian Godwit (L. hae- mastica), Sanderling (Crocethia alba) , and Northern Phalarope (Lobipes Zobatus) were recorded from only the northward migration whereas the Snowy Plover (Charadrius alexandrinus) , Dunlin (Erolia alpina) , and American Avocet (Recurvirostra americana) were seen only during the south- bound season. Despite increased populations following the reproductive season, the Golden Plover (Plzzvialis dominica), Spotted Sandpiper (Actitis macularia) , Greater Yellowlegs (Totunus melunoleucus) , Pectoral Sandpiper (Erolia melunotos) , Bairds’ Sandpiper (E. bairdii) , Buff-breasted Sandpiper (Tryngites subruficoZZis), and Wilsons’ Phalarope (Steganopus tricolor) were all much less common during southward than northward migration. Our coverage of the study area and its apparent attractiveness to shorebirds were relatively constant throughout the year and thus cannot account for this paradox. Conversely, the Western Sandpiper (Ereunetes mauri) was numer- ous during the southward migration, but recorded on only four dates in spring. The Upland Plover (Rartramia longicaudu) , Solitary Sandpiper (Tringa solitariu) , Least Sandpiper (Erolia minutiZZu), Long-billed Dowitcher (Limnodromus scolopaceus), and Stilt Sandpiper (Micropalama himantopus) were also much more common in fall than in spring. Among these, only the Solitary and Western sandpipers showed an increase in numbers greater than SHOREBIRD MIGRATION 169 TABLE 1 RANKING OF SHOREBIRDS BY ABUNDANCE IN NORTHWARD MIGRATION, 1961-63 Maximum Species Rank Total Total dates individuals daily totals Bairds’ Sandpiper 1 1% 2,553 150 White-rumped Sandpiper 2 86 1,524 105 Semipalmated Sandpiper 3 76 900 145 Buff-breasted Sandpiper 4 46 1,027 215 Wilsons’ Phalarope 5 56 1,071 140 Pectoral Sandpiper 6 117 857 75 Least Sandpiper 7 73 877 75 Lesser Yellowlegs 8 104 848 64 Greater Yellowlegs 9 64 587 50 Golden Plover 10 36 269 35 Stilt Sandpiper 11 31 88 17 Long-billed Dowitcher 12 31 95 11 Upland Plover 13 23 79 15 Spotted Sandpiper 14 39 68 6 Semipalmated Plover 15 14 29 7 Common Snipe 16 18 22 3 Hudsonian Godwit 17 6 14 5 Willet 18 6 13 6 Sanderling 19 6 13 4 Black-bellied Plover 20 2 13 7 Marbled Godwit 21 4 8 3 Western Sandpiper 22 4 5 2 Solitary Sandpiper 23 5 5 1 Long-billed Curlew 25 1 1 1 Whimbrel 25 1 1 1 Northern Phalarope 25 1 1 1 that explainable by the addition of offspring to the population. The Semi- palmated Plover (Charadrius semipalmatus) , Killdeer (C. vociferus) , Com- mon Snipe (Capella g&&ago), Lesser Yellowlegs (Totanus fluvipes) , and Semipalmated Sandpiper (Ereunetes pusillw) were about as common during the northward as during the southward migration. The relative abundances (after Urner and Storer, 1949) of 26 species of shorebirds found in northward migration, and of 23 species seen in southward migration are presented in Tables 1 and 2. The Killdeer is excluded from this comparison because of its year-round presence. Seasonal extreme dates for the three-year span are given below for each species. Migration peaks and dates of maximum counts for both northward and southward seasons are given where data are adequate. It must be remembered that our area does not provide optimum habitat for all species of shorebirds. Common Snipe and Solitary Sandpipers are more June 1966 170 THE WILSON BULLETIN Vul.